Mr Ratcliffe told the conference that his wife was “pushed over the edge” to start the strike when, on December 29, investigators asked her to spy on Britain.

“She was told it would be safer for her and safer for her family afterwards if she agreed to do this,” he said.

“She was told to think about it and that they would return. She has been terrified ever since.”

Specific demands were made to snoop on the Department for International Development and London-based organisation Small Media, he added.

Today I summoned the Iranian Ambassador to demand Nazanin has immediate access to the healthcare she requires. Her ongoing detention is TOTALLY unacceptable and her treatment at the hands of Iranian authorities is a fundamental breach of human rights 1/2

The escalation – not yet ruled out by the Foreign Secretary – would give the UK greater access to the charity worker.

Mr Ratcliffe said the minister vowed to “do all we can” to return his wife.

The pair spoke after Mr Hunt summoned the Iranian ambassador to the UK for a meeting. It is the first time he has made such a move since becoming Foreign Secretary.

Mr Hunt tweeted: “Her ongoing detention is TOTALLY unacceptable and her treatment at the hands of Iranian authorities is a fundamental breach of human rights.

“It is a truly terrible indictment of Iran’s approach that she feels she needs to resort to such an ordeal. Iran must take action now.”

Ms Zaghari-Ratcliffe, who works for the Thomson Reuters Foundation, was arrested at Tehran’s Imam Khomeini airport on April 3 2016 and sentenced to five years in jail.

Iran has publicly accused her of spying, which she vehemently denies.

Ms Zaghari-Ratcliffe was planning to go on an initial three-day hunger strike alongside fellow Evin prison detainee Narges Mohammedi – but may fast for longer if her demands are not met.

The charity worker was jailed for five years on charges of spying (Handout/PA)

Human rights charity Redress was among those calling for diplomatic protection to be granted.

A Foreign Office spokeswoman said: “The Foreign Secretary discussed this course of action with Richard Ratcliffe today, and will continue to actively consider the question of diplomatic protection alongside all other options in Nazanin’s case and the cases of other dual nationals detained in Iran.

“The Foreign Secretary has been clear that innocent people in prison must not be used as diplomatic leverage.”

Richard Ratcliffe on this ordeal with Iran's authorities: “One of the most ominous parts was the pressures on Nazanin’s family. That’s one of the reasons we made these things public – to stop them playing with us.” #FreeNazanin